1. [Field of the Invention]
The present invention relates to a torque detector for detecting torque without contact when external force is applied to a rotary shaft in a car power steering system or the like.
2. [Description of the Prior Art]
In a car power steering system, it is necessary to detect torque applied to a steering wheel so as to determine the amount of power assist. A torque detector for this purpose is disclosed by Japanese Laid-open Utility Patent No. 1-180737. The structure of this torque detector will be described with reference to FIG. 6. In FIG. 6, reference numeral 1 denotes a first shaft connected to an unshown steering wheel, 2 a second shaft connected to the unshown pinion gear of a steering unit, 3 a torsion bar for elastically connecting the two shafts in a torsion direction, 4 a cylindrical case, 5 a bobbin stored in the case, 6 bearings for fixing the shaft 1 in the case 4 in such a manner that the shaft 1 can rotate freely, 11 and 12 first and second cylinders made from a soft magnetic material and fixed to the first shaft 1, 13 and 14 third and fourth cylinders made from a soft magnetic material and fixed to the second shaft 2, 11a to 14a tooth portions provided on the first to fourth cylinders, and 21a to 21d first to fourth coils wound in the bobbin 5.
A description is subsequently given of the operation of this torque detector. When torque is applied to the first shaft 1 from the steering wheel, the torsion deformation of the torsion bar 3 occurs and a relative angular difference is generated between the first shaft 1 and the second shaft 2. Then, relative displacement between the first cylinder 11 and the third cylinder 13 fixed to the respective shafts occurs in a circumferential direction, and the area forming a magnetic path between the opposite tooth portions 11a and 13a provided on the respective cylinders is changed, resulting in a variation in the inductance of the first coil 21a. The torque can be obtained by detecting this variation in the inductance by means of an unshown detection circuit.
However, as the inductance of the first coil 21a is changed not only by torque but also by temperature, the third coil 21c whose inductance is changed not by the torsion of the torsion bar but by temperature only in the same manner as the first coil 21a is provided and temperature compensation is made by detecting the inductance of the third coil 21c.
As described above, torque on which temperature compensation has been made can be measured with the first detection set comprising the first and third cylinders and the first and third coils. However, the torque detector of this prior art comprises a second detection set.
Since the second cylinder 12 and the fourth cylinder 14 are fixed to the first shaft 1 and the second shaft 2, respectively, like the first detection set, they constitute the second detection set with the second and fourth coils, and the second detection set operates in the same manner as the first detection set. Therefore, the torque detector has a double safety function which enable the system to operate with the output of the other detection set when a failure such as disconnection occurs in either one of the detection sets.